1. Field of the Invention
The present invention relates to equipment and methods for coating and electroplating objects. More particularly, this invention relates to a method and apparatus for coating and electroplating objects such as printed circuit boards.
2. Description of the Related Art
The plating of objects, such as printed circuit boards (PCBs), may be accomplished by various processes. For PCBs having holes, an initial step or process is often required to coat or plate the holes so as to provide a conductive electrical path on the hole surfaces.
This metal deposition step, often called “electroless plating,” does not rely on the typical electric fields that are associated with electroplating processes. Therefore, this metal deposition step is generally considered harder to regulate than a regular electroplating process. Thus, rather than use such an electroless plating technique to fully plate an object such as a PCB, after a base of conductive metal plating is applied by electroless means, the object is moved to an electroplating process for greater speed and control of the further addition of conductive material to the surfaces of the object.
In a conventional metal deposition process, the object to be coated is immersed in various chemical solutions. For PCBs, this usually involves positioning the PCBs vertically (i.e., with their printed surfaces and the direction of the axis of their holes facing sideward), clamping them at an upper or lower edge, and lowering them into a open-top tank for treatment, then raising them from such tanks after the treatment is complete. As a complete coating usually will require a number of treatments, including rinsing, coating, washing, etc., the PCBs usually have to be transported from tank to tank of different chemical solutions.
Such tanks can often take up considerable floor space, require large quantities of chemicals to fill them to the depths necessary to allow the PCBs to be fully immersed and often require sizable ventilation hoods and air cleaning systems to address the safety concerns associated with utilizing such chemicals in open-top tanks. These factors are some of the disadvantages of such metal deposition processes.
One of the most commonly used electroplating processes for adding conductive material to such coated objects is also an immersion type of process. It involves an open-top tank filled with plating solution or electrolyte. Electrodes are placed in the tank, sometimes referred to as a plating bath, and a voltage is applied to the electrodes to set up an electric field in the tank. A power supply that has a variable voltage and amperage control is used to create the voltage between the electrodes. One electrode, called the anode, is attached to the positive terminal of a power supply. The other electrode, called the cathode, is attached to the negative or ground terminal of the power supply. The object to be plated is connected to the negative side of the power supply and, in effect, the object becomes the cathode side of the circuit. The plating material will deposit on the cathode side of the circuit.
The electrolyte consists of a chemical solution where the plating material, usually a metal, is dissolved and suspended in the liquid as metal ions. When the electric field is applied, the metal ions deposit on the cathode and increase its thickness over time until the process is halted or the solution is exhausted. If the liquid is quiescent with no liquid currents or agitation, the electrolyte in the liquid layer immediately adjacent to the plated surface will become depleted of ions. The rate of metal ion deposition will slow down by an order of magnitude unless the electrolyte is agitated or some other means is used to deliver fresh electrolyte. Agitation is also used to dislodge and sweep away bubbles that sometimes form on the plating surface. The bubbles, typically hydrogen, form as a consequence of the deposition process and the bubbles will obstruct plating and create pinholes and non-uniform thickness in the plating.
In such an electroplating process, the objects to be plated, e.g., PCBs, are also usually positioned vertically, clamped on an edge and lowered into an open-top tank of electrolyte for treatment. As with metal deposition, such an electroplating process will often include a number of steps (e.g., rinsing, plating, etching, washing), with the objects to-be-plated being transported from tank to tank of differing, specified solutions.
Thus, it often proves to be the case that such electroplating processes have the same disadvantages as previously seen for the metal deposition process, i.e., their tanks take up considerable floor space, require large quantities of chemicals to fill them to the depths necessary to allow the PCBs to be fully immersed and often require sizable ventilation hoods and air cleaning systems to address the safety concerns associated with utilizing such electrolytic solutions in open-top tanks.
Because of these disadvantages, there exists a continuing need for improved metal deposition and electroplating methods and apparatus.
b 3. Objects and Advantages
There has been summarized above, rather broadly, the prior art that is related to the present invention in order that the context of the present invention may be better understood and appreciated. In this regard, it is instructive to also consider the objects and advantages of the present invention.
It is an object of the present invention to provide a method and apparatus that yields more efficient and lower cost metal deposition and electroplating of objects, e.g., PCBs.
It is an object of the present invention to provide a method and apparatus for metal deposition and electroplating of objects that requires less floor space than that required by prior art methods and apparatus utilizing open-top tanks.
It is an object of the present invention to provide a method and apparatus for metal deposition and electroplating of objects that requires lesser quantities of chemicals than that required by prior art methods and apparatus utilizing open-top tanks.
It is an object of the present invention to provide a method and apparatus for metal deposition and electroplating of objects that requires smaller-size air cleaning systems than that required by prior art methods and apparatus utilizing open-top tanks.
These and other objects and advantages of the present invention will become readily apparent as the invention is better understood by reference to the accompanying summary, drawings and the detailed description that follows.